Magnetic experiments, microscopic analyses and X-ray diffractometry were conducted on oxidation products of Mn-bearing natural crystalline siderite after successive heating steps in air. Magnetic measurements indicated the production of single-domain and pseudo single domain ferrimagnetic phases with Curie temperatures between 420¿C and 560¿C. Hematite formation was detected by X-ray diffraction after heating the siderite samples at 480¿C, while Mn-ferrite was formed at 500¿C and persisted even after heating to 700¿C. This final product is stable with a spinel structure and unit cell parameters intermediate between those of magnetite and maghemite. Transmission electron microscopy and electron energy loss spectroscopy analyses confirmed the presence of Mn in the spinel ferrite phase, which contains Fe(III), Mn(III), and Mn(II) ions. This phase appears as single crystals with euhedral shapes of average grain size 90--100 nm. In comparison to pure magnetite, the addition of Mn in the crystal lattice lowers the Curie temperature and explains the stability of the spinel phase. Thus stable magnetization carried by a magnetite related phase can be generated by oxidation of Mn-siderite.